The present invention relates to a microstructured optical fibre with cladding recess, preferably a microstructured optical fibre exhibiting a high numerical aperture; a method of its production; and an apparatus comprising a length of such a microstructured fibre, preferably a side-pumped optical fibre laser or optical fibre amplifier.
1. The Technical Field
Today optical fibres are mainly used in point to point communication were the power transmitted remains modest, rarely exceeding 100 mW. But with the deployment of metropolitan networks large saturation power amplification is sought, to allow for large signal splitting ratios at nodes. The advent of high power is also expected to open new markets to fibre lasers such as marking, thermal printing, inter-satellite communication, and material processing.
In recent years the power delivered by laser diodes in form of single stripes, bars, or arrays has increased rapidly.
Because of their compactness, high efficiency (electrical-to-optical power conversion in excess of 40%), high reliability, and steadily decreasing price/watt they are progressively replacing other laser systems. But these powerful sources (several W to several 100 W) are highly multimode. This limits the spot size that can be achieved at focus. In most applications their brightness (or radiance) needs to be significantly enhanced.
2. Prior Art Disclosures
A particularly efficient technique first suggested by Snitzer et al., the so-called cladding pumping technique, is now widely used, see for example EP 88121151. This technique makes use of a double-clad optical fibre, hereinafter called standard optical fibre. Snitzer et al. discloses a double clad fibre with a small core, typically a few micrometers in diameter that is single-mode at an emission wavelength. The core is doped with active elements such as rare earth elements, and it is embedded in—or surrounded by—an inner cladding that acts as a (large) multimode waveguide. The pump is launched into the inner cladding, where after it is absorbed in the core and re-emitted at a signal wavelength from the core—thereby enabling lasing or amplification to take place in the core.
Recently, using a cladding pumping technique, a research team at SDL Corporation demonstrated more than 100 W of output power in the fundamental mode from an ytterbium fibre laser, see Dominic et al. CLEO'99, Conference on Lasers and Electro Optics, CPD11-1, 1999. Despite the very high intensity in the embedded single mode core, in excess of 100 MW/cm2, no damage to the fibre was observed, and the output power was only limited by the amount of light that could be launched into the fibre. Dominic et al. used 4*45 W diode packages to pump each end of the fibre, where two sources were polarization combined. For pumping the fibre with more sources and reaching larger output powers other pumping schemes are desired. Moreover, access to the signal is rendered difficult by the end-pumping scheme. This is particularly problematic in an optical amplifier configuration. Recognizing the need for a more flexible pumping scheme several groups have proposed alternatives to end pumping, such alternatives include accessing an active fibre through its side.
Valentin et al. in U.S. Pat. No. 5,999,673 disclose optical pumping of standard optical fibres from the side. Other examples include Heflinger et al. in U.S. Pat. No. 6,243,515, and Ionov et al. in U.S. Pat. No. 6,317,537 B1. Ionov et al. disclose a method and apparatus for coupling substantial optical power into a standard optical fibre from the side without encumbering ends of the fibre. However, prior art side-pumped optical fibres generally do not have an efficient coupling of pump light, e.g. of pump light from high power sources.
Recently, a new type of optical fibres comprising microstructured features has been demonstrated—see for example Broeng et al., Optical Fiber Technology, Vol. 5, pp. 305–330, 1999 or DiGiovanni et al. U.S. Pat. No. 5,907,652. In U.S. Pat. No. 5,907,652, DiGiovanni et al. discloses optical fibres comprising a microstructured outer cladding region, an inner cladding region and a central core.
Furusawa et al., in Optics Express, Vol. 9, No. 13, pp. 714–720, 2001 disclose a cladding-end-pumped microstructured fibre laser having a holey outer cladding and exhibiting a NA in the range 0.3 to 0.4. Nothing is indicated nor suggested about side-pumping a microstructured fibre.
Russell et al., WO 0142829, disclose microstructured fibres having a recess for evanescent field sensing. The recess provides physical access to the exposed core of the microstructured fibre, i.e. the core is exposed to the outside environment, thereby allowing evanescent field sensing using light propagating in a core of the microstructured fibre. However, for applications of microstructured fibres in devices, or apparatus, such as for example lasers or amplifiers, the physical access to the core causes light leakage from the core and thereby degrades amplified light signal or a lasing mode in the fibre core.